Photovoltaic (PV) solar energy production has been of interest for several decades, and with consistent decreases in the cost of solar arrays, along with a growing cost of non-solar energy, the penetration of PV energy sources is expected to continue to rise. Interest in PV and other systems in reducing greenhouse gas emissions and improving air quality can also be expected to raise the demand for PV energy sources.
In order to use PV energy sources in conjunction with existing power grids (“mains” supplies), it is necessary to convert the DC output of the PV source to AC current at the mains frequency and power. This conversion is typically accomplished by an inverter circuit that converts the output of the PV array to the necessary single phase or multi-phase AC current(s). It is desirable to operate the PV energy source at a load (output) current level determined from a maximum power point (MPP) of the PV source, which varies with the amount of available sunlight. Operating at the MPP provides the most efficient use of the PV energy source, as long as all of the energy can be transferred to the mains supply. The voltage of the PV source varies with the load current level, but the MPP voltage is substantially constant, while the MPP current level changes with the available sunlight. The inverter may directly convert the DC output of a PV array to, for example, a 230VRMS two-phase power main at 60 Hz for the United States for utility customer owned PV applications, or a three-phase power main at 60 Hz and a suitable voltage for operating a transformer to step-up the inverter output to a distribution system voltage such as 4160VRMS.
More efficient inverters have been implemented that use a cascaded configuration to convert the output of the PV energy source to a high-voltage DC source, such as 360V, using a DC-DC converter, and then generate the AC mains supply output using an AC inverter that operates from the 360V DC power supply generated by the DC-DC converter. However, such inverters require large capacitors for the high-voltage DC source in order to prevent ripple due to the varying current at the input of the AC inverter, which would otherwise be present on the output of the DC-DC converter. If the ripple is not filtered, the variation in voltage at the output of the DC-DC converter causes large variations in the load current drawn from the PV energy source, which represent deviation from the MPP and therefore a drop in system efficiency.
Large electrolytic capacitors are undesirable both from a cost and volume standpoint, but in PV array applications, they are also undesirable for their impact on the reliability of the system. With system life-spans exceeding 30 years and with operational requirements at high temperatures, the use of electrolytic capacitors has a large impact on the mean-time-between-failure (MTBF) of the power converter, and thus a PV system as a whole.
Therefore, it would be desirable to provide a power converter for coupling a PV energy source to a mains supply while maintaining PV energy source operation substantially at the MPP and reducing variation in the PV energy source loading without requiring large unreliable electrolytic capacitors.